The phrase "geosynthetic material" is broadly used to refer to a large class of engineered products that are used in a variety of geostructural applications including soil stabilization, support for earthworks, erosion barriers and retaining walls, among others. The phrase "geosynthetic material" may refer to structures or to the basic components of structures. The phrase "geosynthetic materials" as used herein does not include materials employed in the construction of buildings or materials used as an interlayer in the construction of concrete and/or asphalt roadways or roadway patch materials, as these are not geostructures, i.e. earthen structures or structures used to control or reinforce earthen structures.
The applications where geosynthetic materials are often employed may be broadly classified into six major functions: reinforcement of soil; separation of soil layers; soil filtration; controlled drainage; erosion control; and providing moisture barriers in or about soil. Soil reinforcement refers generally and broadly to increasing tensile and/or shear strength of earth or particulate structures, such as in retaining wall structures, steep grades and other applications that compel tensile and/or shear strength enhancement of particulate substrate properties. Some of the types of geosynthetic materials that perform these functions include: 1) geotextiles (also known as "geotextile fabrics" or "geofabrics", which include interwoven, non-interwoven or nonwoven fabric-like materials generally for separation/reinforcement); 2) geogrids (sometimes considered a subclass of geotextiles and which include grid-like structures having relatively large grid openings therein, generally for soil reinforcement); 3) geomembranes (sheet-like materials having little or no permeability to moisture, generally for moisture barrier applications); 4) geosynthetic clay liners (liners often consisting of a layer of bentonite or other very low permeability material supported by geotextiles, geogrids or geomembranes generally for moisture barrier applications); 5) erosion control products (any number of fabric-like, grid-like or sheet-like materials used to restrain the movement of soil or other components of particulate substrates, whether by wind, water or otherwise); and 6) specialty geosynthetics (generally referring to geosynthetics not otherwise classified).
The presently available geosynthetic materials, particularly geotextiles and geogrids, are predominately formed from polymeric materials. For example, several polymeric geogrids are available from Strata Systems, Inc. of Cumming, Ga., as described in technical sales brochures entitled: Strata Systems. Inc., A Better Way to Build; STRATAGRID 100; STRATAGRID 200; STRATAGRID 300; STRATAGRID 400; STRATAGRID 500; STRATAGRID 600 and STRATAGRID 700. The Strata Systems, Inc. geogrids are manufactured from polyester yarns knitted by warp knit weft insertion into a grid-like structure having a uniform network of apertures and providing tensile reinforcement in one principal direction. A polymeric coating, (e.g. a polyvinyl chloride coating) provides additional mechanical as well as chemical and ultraviolet radiation degradation protection.
Also for example, U.S. Pat. No. 5,669,796 discloses a geogrid comprised of bicomponent fibers comprising a polyethylene terephthalate core within a sheath of a polyolefin and including carbon black for ultraviolet (UV) stabilization. The grid is a warp knit, weft inserted geogrid in which the fibers are knit into a fabric and heat bonded together. The bicomponent fibers are described as providing an improved resistance to creep. The grid is not topcoated with coatings such as polyvinyl chloride (PVC) topcoats. Avoiding the topcoating process is described therein as beneficial in reducing manufacturing costs and reducing potential environmental problems.
Also for example, U.S. Pat. Nos. 4,421,439, 4,837,387 and 5,187,004 describe a supporting fabric, primarily for supporting soil materials. The fabric is a tri-layered non-coplanar grid of synthetic warp and weft yarns having limited fabric elongation or, in the case of U.S. Pat. No. 5,187,004, an ability to support chemically aggressive materials, particularly soil materials. The warp yarns are described as being formed of polyester and polyethylene terephthalate. Other polymeric yarns are listed as acceptable alternatives. The references describe the weft yarns as being made of the same material as the warp yarns or of a different material. An example is given of the combination of polyester warp yarns with polypropylene weft yarns.
As a further example, U.S. Pat. Nos. 4,960,349 an 5,091,247 describe an interwoven geotextile grid. The grid is formed of a plurality of spaced apart polymeric pick yarn bundles interwoven with a plurality of spaced-apart polymeric warp yarn bundles. A plurality of pairs of leno yarns parallel to the warp yarns add additional strength to the fabric, as do polymeric locking yarns. The grid is coated with a suitable PVC or other plastic coating such as latex, urethane or polyethylene coatings.
An erosion control mat which includes a grid-like scrim having a web of unconsolidated fibers disposed thereon is disclosed in U.S. Patent Nos. 5,249,893 and 5,358,356. The scrim and web are described as being formed of polypropylene, polyester, nylon, rayon, polyethylene, cotton or combinations of any two or more thereof.
U.S. Pat. No. 4,472,086 discloses a fabric which includes a grid composed of a first group of synthetic threads arranged substantially transversely to a second group of synthetic threads, wherein the first and second groups of threads are bonded to each other by knit yarn stitch bonds. According to the reference, because the fabric does not include an adhesive found in many fabrics, the basic yarn elongation is the only factor affecting fabric elongation, so that fabric elongation is precisely controllable. At column 4, lines 10-12, the reference states that the preferred synthetic material is a polyester or polypropylene. The fabric of the reference is used as an intermediate between a cracked road surface and an asphalt patch to be placed over the crack, wherein the fabric operates to prevent reflective racks from reflecting from the cracked road surface and into and through the asphalt patch.
Additional examples of polymeric based geosynthetic materials may be found in U.S. Pat. Nos. 4,374,798, 4,610,568, 4,662,946, 4,756,946, 4,851,277, 5,156,495, 5,419,659, 5,567,087, and 5,651,641.
One important limitation common among polymerically based geosynthetic materials, particularly geogrids, is that such materials are subject to substantial strain. Strain refers to the elongation of the geosynthetic material under tensile load, generally normalized with respect to cross-sectional area. Depending upon the orientation of the tensile load with respect to the geosynthetic material, the strain may occur along the longitudinal, transverse or both directions of the geosynthetic material. Strain resulting in 5 to 30 percent or more elongation of polymeric geosynthetic material is not uncommon, even at tensile loads which are only about 20 to 50 percent of the short term ultimate strength of the polymeric geosynthetic material. Another limitation common among polymerically based geosynthetic materials, particularly geogrids, is that such materials are subject to creep. Creep refers to the elongation of the geosynthetic material under a sustained tensile load. Yet another drawback of many polymeric based geosynthetic materials is that they deteriorate when subjected to ultraviolet radiation, either limiting the durability of such materials or requiring additional coatings and the like to protect the geosynthetic materials from the adverse effects of the ultraviolet radiation. A still further drawback of many polymerically based geosynthetic materials is that the weight of such materials per unit area is substantial, particularly where the material has been designed to withstand substantial loads, making such materials challenging to transport and install.
Grid-like structures formed of materials other than polymerically based materials are known, but such structures are not commonly used as geosynthetic materials. For example, U.S. Pat. Nos. 4,699,542, 4,957,390, 5,110,627, 5,246,306, and 5,393,559 generally describe reinforcements for asphaltic pavings comprising a grid of continuous glass fibers stitched at intersections of the crosswise and lengthwise strands to hold the grid shape. The grid may be overcoated with an asphaltic or resin coating to impart a semi-rigid nature to the grid. The resins may be selected from asphalt, rubber, modified asphalt, unsaturated polyesters, vinyl ester, epoxy, polyacrylates, polyurethanes, polyolefins and phenolics. The grid is laid on top of an underlying paving and adhered to it and an asphaltic paving layer is then applied on top of the grid.
U.S. Pat. Nos. 4,491,617, 4,539,254, 4,762,744, 4,780,350, 5,439,726, disclose the use of similar grid-like structures in bituminous roofing membranes.
U.S. Pat. No. 5,552,207 discloses an open grid fabric for reinforcing wall systems such as stucco walls, where the grid is affixed to a supporting wall such as a foam insulation board, and is then overcoated with the stucco material. The open grid fabric improves impact resistance for durability. The grid is formed of first and second sets of substantially parallel rovings which are combined using certain knits, leno weaves or adhesive methods. The rovings are direct-sized with at least a silane sizing. Warp rovings and weft rovings are tied together in a knitting process by a tie yarn. Preferred warp rovings and weft rovings are fiberglass strands, but others such as nylon, aramid, polyolefin and polyester may be used in various combination. The tie yarn is described as typically a low weight polyester, however the tie yarn may be formed from other materials as listed at column 7, lines 43-48 of the reference. The rovings of the open grid fabric are further locked together by a polymeric resin, such as polyvinyl chloride, polyvinylidene chloride, styrene butadiene rubber, urethane, silicone, acrylic and styrene acrylate polymers.
An interwoven fabric of glass fiber or other inorganic warp and weft in which one or more selected warp ends are secured at each weft crossover by a bond of thermoplastic material, suitably nylon or polyester, is disclosed in U.S. Pat. No. 3,515,623. The thermoplastic material is melted to bond the crossovers together and prevent unraveling of the interwoven fabric. Such fabric is used, according to Bates et al., as internal reinforcing mesh in plied roofing papers or heavy duty wrapping papers and the like.
A technical bulletin of PPG Industries, Inc. of Pittsburgh, Pa., entitled "HERCUFLEX.TM. Strand: The Applications Are Endless", (about 1990), which is hereby incorporated by reference, suggests the use of HERCUFLEX.TM. fiberglass strand for geotextiles.
Impregnated Fiber-Glass Yarn For High-Strength Geosynthetic Reinforcement, Girgis, M., High-Tech Fibrous Materials, Chapter 22, pp. 337-350, American Chemical Society, Washington, D.C. (1991) describes the use of fiber-glass yarn for geosynthetic reinforcement. See also, Impregnated Fiber Glass Yarns For Reinforcing Industrial Coated Fabric, Girgis, M., J. of Coated Fabrics, Vol. 17, pp. 230-241 (April 1988) which describes the use of polymerically coated glass fibers in geotextile applications, among others.
Walls Reinforced With Fiber Plastic Geogrids In Japan, Miyata, K., Geosynthetics International Vol. 3, No. 1, pp. 1-11 (1996) describes geosynthetic reinforced soil retaining walls reinforced with high tensile strength and stiffness fiber reinforced plastic using a geogrid produced by impregnating high tensile strength continuous glass fiber bundles with vinyl-ester resin which was then molded to give the required geogrid geometry.
U.S. Pat. No. 4,990,390 describes a fiber grid reinforcement which includes a plurality of first fiber bundles and a plurality of second fiber bundles which perpendicularly intersect the plurality of first fiber bundles to form a grid. The fibers in each bundle and fiber bundles are bound to one another by a resin material. Column 3, lines 39-40 of the reference indicate that the fibers may be selected from glass fibers, and at lines 47-50 indicate that the resin can be a vinyl ester resin, unsaturated polyester resin, epoxy resin, phenol resin, among others.
U.S. Pat. No. 5,007,766 to Freed describes a shaped barrier for erosion control and sediment collection which includes a plurality of strands emanating outwardly from a foundation common to all of the strands, wherein the strands may be formed of glass, among other substances.
Chemically treated fibers, including glass fibers, and fabrics made thereof are described in U.S. Pat. Nos. 4,390,647, 4,663,231, 4,762,750, 4,762,751, and 4,795,678.
Despite the foregoing, most engineering fabrics or geotextiles in widespread use today are made from polymeric materials or fibers. It would be advantageous to provide a geosynthetic material which is not polymerically based and which does not suffer from substantial strain and/or creep, ultraviolet radiation sensitivity, weight per unit area, and/or the biological/chemical sensitivity common to some of the polymerically based geosynthetic materials presently available.